Sequential vs parallel execution in Javascript - Visual Side-by-Side Comparison

Concept Flow - Sequential vs parallel execution

Start

↓

Task 1

↓

Task 2

↓

Task 3

↓

End

↓

Start

↓

Task1

↓

Task3

↓

End

Sequential runs tasks one after another. Parallel runs tasks at the same time.

Execution Sample

Javascript

console.log('Start');
setTimeout(() => console.log('Task 1'), 1000);
setTimeout(() => console.log('Task 2'), 500);
console.log('End');

Shows how JavaScript runs code sequentially but schedules tasks to run later in parallel.

Execution Table

Step	Action	Output	Notes
1	console.log('Start')	Start	Runs immediately
2	setTimeout Task 1 scheduled		Task 1 will run after 1000ms
3	setTimeout Task 2 scheduled		Task 2 will run after 500ms
4	console.log('End')	End	Runs immediately after scheduling
5	After 500ms, Task 2 runs	Task 2	Runs before Task 1 because shorter delay
6	After 1000ms, Task 1 runs	Task 1	Runs last
7	All tasks done		Execution complete

💡 All scheduled tasks have run; no more code to execute.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
Task 1 Timer	undefined	Scheduled (1000ms)	Scheduled (1000ms)	Executed
Task 2 Timer	undefined	Scheduled (500ms)	Scheduled (500ms)	Executed

Key Moments - 2 Insights

Why does 'End' print before 'Task 1' and 'Task 2'?

Because console.log('End') runs immediately in the main thread, while setTimeout schedules tasks to run later asynchronously (see execution_table steps 4 and 5).

Why does 'Task 2' print before 'Task 1' even though Task 1 was scheduled first?

Because Task 2 has a shorter delay (500ms) than Task 1 (1000ms), so it runs first after the delay (see execution_table steps 5 and 6).

Visual Quiz - 3 Questions

Look at the execution_table, what is printed at Step 4?

A Task 1

B Start

C End

D Task 2

💡 Hint

Check the 'Output' column for Step 4 in the execution_table.

At which step does 'Task 2' run?

A Step 2

B Step 5

C Step 4

D Step 6

💡 Hint

Look for when 'Task 2' appears in the 'Output' column in the execution_table.

If both setTimeout delays were 0ms, what would change in the execution_table?

A Task 1 and Task 2 would run after 'End' but at the same time.

B Task 1 and Task 2 would run immediately before 'End'.

C 'End' would print last.

D No change; order stays the same.

💡 Hint

Remember setTimeout with 0ms delay still runs after current code finishes (see variable_tracker and execution_table timing).

Concept Snapshot

Sequential execution runs code line by line.
Parallel execution runs tasks at the same time.
JavaScript uses event loop to handle parallel tasks.
setTimeout schedules tasks to run later asynchronously.
Immediate code runs before scheduled tasks.
Tasks with shorter delay run earlier.

Full Transcript

This visual shows how JavaScript runs code sequentially but schedules some tasks to run later in parallel using setTimeout. The code prints 'Start', schedules two tasks with different delays, then prints 'End'. The tasks run after their delays, so 'Task 2' prints before 'Task 1' because it has a shorter delay. Variables track the timers scheduled and when they execute. Key moments clarify why 'End' prints before tasks and why task order depends on delay. The quiz tests understanding of output order and timing. This helps beginners see how sequential and parallel execution work together in JavaScript.

Practice

1.

What is the main difference between sequential and parallel execution in JavaScript?

easy

A. Sequential uses promises; parallel uses only callbacks.

B. Sequential runs tasks one after another; parallel runs tasks at the same time.

C. Sequential runs tasks at the same time; parallel runs tasks one after another.

D. Sequential is faster than parallel always.

Solution

1. Step 1: Understand sequential execution

   Sequential execution means tasks run one by one, waiting for each to finish before starting the next.

2. Step 2: Understand parallel execution

   Parallel execution means tasks run at the same time, so they can finish faster by overlapping.

3. Final Answer:

   Sequential runs tasks one after another; parallel runs tasks at the same time. -> Option B

4. Quick Check:

   Sequential = one by one, Parallel = at the same time [OK]

Hint: Remember: sequential = one by one, parallel = together [OK]

Common Mistakes:

• Confusing which runs tasks simultaneously
• Thinking sequential is always faster
• Mixing up async/await with execution type

2.

Which of the following is the correct syntax to run two promises in parallel using Promise.all?

const p1 = fetch(url1);
const p2 = fetch(url2);
// What is correct?

easy

A. const results = await Promise.race([p1, p2]);

B. const results = await Promise.parallel([p1, p2]);

C. const results = await Promise.sequence([p1, p2]);

D. const results = await Promise.all([p1, p2]);

Solution

1. Step 1: Identify correct Promise method for parallel

   Promise.all runs multiple promises in parallel and waits for all to finish.

2. Step 2: Check syntax correctness

   Using await Promise.all([p1, p2]) correctly waits for both promises in parallel.

3. Final Answer:

   const results = await Promise.all([p1, p2]); -> Option D

4. Quick Check:

   Promise.all = parallel wait [OK]

Hint: Use Promise.all for parallel waits, not race or sequence [OK]

Common Mistakes:

• Using Promise.race which waits only for first promise
• Using non-existent Promise.sequence or Promise.parallel
• Not wrapping promises in an array

3.

What will be the output of this code?

async function test() {
  console.log('Start');
  await new Promise(r => setTimeout(r, 1000));
  console.log('Middle');
  await new Promise(r => setTimeout(r, 1000));
  console.log('End');
}
test();

medium

A. Start immediately, then Middle after 1s, then End after 2s

B. Start, Middle, and End all print immediately

C. Start, End, then Middle after 2 seconds

D. Only Start prints, then program stops

Solution

1. Step 1: Analyze first await delay

   After printing 'Start', the code waits 1 second before printing 'Middle'.

2. Step 2: Analyze second await delay

   After 'Middle', it waits another 1 second before printing 'End'.

3. Final Answer:

   Start immediately, then Middle after 1s, then End after 2s -> Option A

4. Quick Check:

   await pauses sequentially, so output is delayed stepwise [OK]

Hint: await pauses code, so logs appear one after another [OK]

Common Mistakes:

• Thinking all logs print immediately
• Confusing setTimeout with immediate execution
• Ignoring await pauses

4.

Find the error in this code that tries to run two async tasks in parallel:

async function runTasks() {
  const result1 = await task1();
  const result2 = await task2();
  return [result1, result2];
}

medium

A. Cannot return array from async function.

B. Missing async keyword on task1 and task2.

C. Tasks run sequentially, not in parallel.

D. await cannot be used inside async functions.

Solution

1. Step 1: Check how tasks are awaited

   Each await waits for task1 to finish before starting task2, so tasks run one after another.

2. Step 2: Identify parallel execution fix

   To run in parallel, start both tasks without await, then await both together using Promise.all.

3. Final Answer:

   Tasks run sequentially, not in parallel. -> Option C

4. Quick Check:

   Separate awaits cause sequential execution [OK]

Hint: Awaiting one by one is sequential, start all then await all for parallel [OK]

Common Mistakes:

• Assuming await runs tasks in parallel
• Thinking async keyword fixes parallelism alone
• Believing async functions can't return arrays

5.

You want to fetch data from three URLs and process results only after all are done. Which code correctly runs fetches in parallel and waits for all?

const urls = ['url1', 'url2', 'url3'];
async function fetchAll() {
  // Fill this
}

hard

A. const results = await Promise.all(urls.map(url => fetch(url)));

B. const results = [];
for (const url of urls) {
results.push(await fetch(url));
}

C. const results = urls.map(url => await fetch(url));

D. const results = await urls.forEach(url => fetch(url));

Solution

1. Step 1: Understand parallel fetch with Promise.all

   Using Promise.all with map starts all fetches at once and waits for all to finish.

2. Step 2: Check other options for errors

   const results = []; for (const url of urls) { results.push(await fetch(url)); } fetches sequentially due to await inside loop; const results = urls.map(url => await fetch(url)); uses await incorrectly inside map; const results = await urls.forEach(url => fetch(url)); misuses forEach which returns undefined.

3. Final Answer:

   const results = await Promise.all(urls.map(url => fetch(url))); -> Option A

4. Quick Check:

   Promise.all + map = parallel fetch [OK]

Hint: Use Promise.all with map for parallel async calls [OK]

Common Mistakes:

• Await inside for loop causing sequential fetch
• Using await inside map callback directly
• Using forEach which does not return promises